תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Oxygen abundances in metal-poor subgiants as determined from [O I], O I and OH lines
The debate on the oxygen abundances of metal-poor stars has its originin contradictory results obtained using different abundance indicators.To achieve a better understanding of the problem we have acquired highquality spectra with the Ultraviolet and Visual Echelle Spectrograph atVLT, with a signal-to-noise of the order of 100 in the near ultravioletand 500 in the optical and near infrared wavelength range. Threedifferent oxygen abundance indicators, OH ultraviolet lines around 310.0nm, the [O i] line at 630.03 nm and the O i lines at 777.1-5 nm wereobserved in the spectra of 13 metal-poor subgiants with-3.0≤[Fe/H]≤-1.5. Oxygen abundances were obtained from theanalysis of these indicators which was carried out assuming localthermodynamic equilibrium and plane-parallel model atmospheres.Abundances derived from O i were corrected for departures from localthermodynamic equilibrium. Stellar parameters were computed usingT_eff-vs.-color calibrations based on the infrared flux method andBalmer line profiles, Hipparcos parallaxes and Fe II lines. [O/Fe]values derived from the forbidden line at 630.03 nm are consistent withan oxygen/iron ratio that varies linearly with [Fe/H] as[O/Fe]=-0.09(±0.08)[Fe/H]+0.36(±0.15). Values based on theO i triplet are on average 0.19±0.22 dex(s.d.) higher than thevalues based on the forbidden line while the agreement between OHultraviolet lines and the forbidden line is much better with a meandifference of the order of -0.09±0.25 dex(s.d.). In general, ourresults follow the same trend as previously published results with theexception of the ones based on OH ultraviolet lines. In that case ourresults lie below the values which gave rise to the oxygen abundancedebate for metal-poor stars.
| Li and Be depletion in metal-poor subgiants
A sample of metal-poor subgiants has been observed with the UVESspectrograph at the Very Large Telescope and abundances of Li and Behave been determined. Typical signal-to-noise per spectral bin valuesfor the co-added spectra are of the order of 500 for the ion{Li}{i} line(670.78 nm) and 100 for the ion{Be}{ii} doublet lines (313.04 nm). Thespectral analysis of the observations was carried out using the Uppsalasuite of codes and marcs (1D-LTE) model atmospheres with stellarparameters from photometry, parallaxes, isochrones and Fe ii lines.Abundance estimates of the light elements were corrected for departuresfrom local thermodynamic equilibrium in the line formation. Effectivetemperatures and Li abundances seem to be correlated and Be abundancescorrelate with [O/H]. Standard models predict Li and Be abundancesapproximately one order of magnitude lower than main-sequence valueswhich is in general agreement with the observations. On average, ourobserved depletions seem to be 0.1 dex smaller and between 0.2 and 0.4dex larger (depending on which reference is taken) than those predictedfor Li and Be, respectively. This is not surprising since the initial Liabundance, as derived from main-sequence stars on the Spite plateau, maybe systematically in error by 0.1 dex or more, and uncertainties in thespectrum normalisation and continuum drawing may affect our Beabundances systematically.
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman
We develop and test a method for the estimation of metallicities([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhancedmetal-poor (CEMP) stars based on the application of artificial neuralnetworks, regressions, and synthesis models to medium-resolution (1-2Å) spectra and J-K colors. We calibrate this method by comparisonwith metallicities and carbon abundance determinations for 118 starswith available high-resolution analyses reported in the recentliterature. The neural network and regression approaches make use of apreviously defined set of line-strength indices quantifying the strengthof the Ca II K line and the CH G band, in conjunction with J-K colorsfrom the Two Micron All Sky Survey Point Source Catalog. The use ofnear-IR colors, as opposed to broadband B-V colors, is required becauseof the potentially large affect of strong molecular carbon bands onbluer color indices. We also explore the practicality of obtainingestimates of carbon abundances for metal-poor stars from the spectralinformation alone, i.e., without the additional information provided byphotometry, as many future samples of CEMP stars may lack such data. Wefind that although photometric information is required for theestimation of [Fe/H], it provides little improvement in our derivedestimates of [C/Fe], and hence, estimates of carbon-to-iron ratios basedsolely on line indices appear sufficiently accurate for most purposes.Although we find that the spectral synthesis approach yields the mostaccurate estimates of [C/Fe], in particular for the stars with thestrongest molecular bands, it is only marginally better than is obtainedfrom the line index approaches. Using these methods we are able toreproduce the previously measured [Fe/H] and [C/Fe] determinations withan accuracy of ~0.25 dex for stars in the metallicity interval-5.5<=[Fe/H]<=-1.0 and with 0.2<=(J-K)0<=0.8. Athigher metallicity, the Ca II K line begins to saturate, especially forthe cool stars in our program, and hence, this approach is not useful insome cases. As a first application, we estimate the abundances of [Fe/H]and [C/Fe] for the 56 stars identified as possibly carbon-rich, relativeto stars of similar metal abundance, in the sample of ``strong G-band''stars discussed by Beers, Preston, and Shectman.
| The lithium content of the Galactic Halo stars
Thanks to the accurate determination of the baryon density of theuniverse by the recent cosmic microwave background experiments, updatedpredictions of the standard model of Big Bang nucleosynthesis now yieldthe initial abundance of the primordial light elements withunprecedented precision. In the case of ^7Li, the CMB+SBBN value issignificantly higher than the generally reported abundances for Pop IIstars along the so-called Spite plateau. In view of the crucialimportance of this disagreement, which has cosmological, galactic andstellar implications, we decided to tackle the most critical issues ofthe problem by revisiting a large sample of literature Li data in halostars that we assembled following some strict selection criteria on thequality of the original analyses. In the first part of the paper wefocus on the systematic uncertainties affecting the determination of theLi abundances, one of our main goal being to look for the "highestobservational accuracy achievable" for one of the largest sets of Liabundances ever assembled. We explore in great detail the temperaturescale issue with a special emphasis on reddening. We derive four sets ofeffective temperatures by applying the same colour {T}_eff calibrationbut making four different assumptions about reddening and determine theLTE lithium values for each of them. We compute the NLTE corrections andapply them to the LTE lithium abundances. We then focus on our "best"(i.e. most consistent) set of temperatures in order to discuss theinferred mean Li value and dispersion in several {T}_eff and metallicityintervals. The resulting mean Li values along the plateau for [Fe/H]≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075when the lowest effective temperature considered is taken equal to 5700K and 6000 K respectively. This is a factor of 2.48 to 2.81 (dependingon the adopted SBBN model and on the effective temperature range chosento delimit the plateau) lower than the CMB+SBBN determination. We findno evidence of intrinsic dispersion. Assuming the correctness of theCMB+SBBN prediction, we are then left with the conclusion that the Liabundance along the plateau is not the pristine one, but that halo starshave undergone surface depletion during their evolution. In the secondpart of the paper we further dissect our sample in search of newconstraints on Li depletion in halo stars. By means of the Hipparcosparallaxes, we derive the evolutionary status of each of our samplestars, and re-discuss our derived Li abundances. A very surprisingresult emerges for the first time from this examination. Namely, themean Li value as well as the dispersion appear to be lower (althoughfully compatible within the errors) for the dwarfs than for the turnoffand subgiant stars. For our most homogeneous dwarfs-only sample with[Fe/H] ≤ 1.5, the mean Li abundances are A(L)_NLTE = 2.177±0.071 and 2.215±0.074 when the lowest effective temperatureconsidered is taken equal to 5700 K and 6000 K respectively. This is afactor of 2.52 to 3.06 (depending on the selected range in {T}_eff forthe plateau and on the SBBN predictions we compare to) lower than theCMB+SBBN primordial value. Instead, for the post-main sequence stars thecorresponding values are 2.260±0.1 and 2.235±0.077, whichcorrespond to a depletion factor of 2.28 to 2.52. These results,together with the finding that all the stars with Li abnormalities(strong deficiency or high content) lie on or originate from the hotside of the plateau, lead us to suggest that the most massive of thehalo stars have had a slightly different Li history than their lessmassive contemporaries. In turn, this puts strong new constraints on thepossible depletion mechanisms and reinforces Li as a stellartomographer.
| The Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Galactic Evolution of Sr, Y, And Zr: A Multiplicity of Nucleosynthetic Processes
In this paper we follow the Galactic enrichment of three easily observedlight n-capture elements: Sr, Y, and Zr. Input stellar yields have beenfirst separated into their respective main and weak s-process componentsand r-process component. The s-process yields from asymptotic giantbranch (AGB) stars of low to intermediate mass are computed, exploring awide range of efficiencies of the major neutron source, 13C,and covering both disk and halo metallicities. AGB stars have been shownto reproduce the main s-component in the solar system, i.e., thes-process isotopic distribution of all heavy isotopes with atomic massnumber A>90, with a minor contribution to the light s-processisotopes up to A~90. The concurrent weak s-process, which accounts forthe major fraction of the light s-process isotopes in the solar systemand occurs in massive stars by the operation of the 22Neneutron source, is discussed in detail. Neither the main s- nor the weaks-components are shown to contribute significantly to theneutron-capture element abundances observed in unevolved halo stars.Knowing the s-process distribution at the epoch of the solar systemformation, we first employed the r-process residuals method to infer theisotopic distribution of the r-process. We assumed a primary r-processproduction in the Galaxy from moderately massive Type II supernovae thatbest reproduces the observational Galactic trend of metallicity versusEu, an almost pure r-process element. We present a detailed analysis ofa large published database of spectroscopic observations of Sr, Y, Zr,Ba, and Eu for Galactic stars at various metallicities, showing that theobserved trends versus metallicity can be understood in light of amultiplicity of stellar neutron-capture components. Spectroscopicobservations of the Sr, Y, and Zr to Ba and Eu abundance ratios versusmetallicity provide useful diagnostics of the types of neutron-captureprocesses forming Sr, Y, and Zr. In particular, the observed [Sr, Y,Zr/Ba, Eu] ratio is clearly not flat at low metallicities, as we wouldexpect if Ba, Eu and Sr, Y, Zr all had the same r-processnucleosynthetic origin. We discuss our chemical evolution predictions,taking into account the interplay between different processes to produceSr-Y-Zr. Making use of the very r-process-rich and very metal-poor starslike CS 22892-052 and CS 31082-001, we find hints and discuss thepossibility of a primary process in low-metallicity massive stars,different from the ``classical s-process'' and from the ``classicalr-process'' that we tentatively define LEPP (lighter element primaryprocess). This allows us to revise the estimates of the r-processcontributions to the solar Sr, Y, and Zr abundances, as well as of thecontribution to the s-only isotopes 86Sr, 87Sr,and 96Mo.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Spectroscopic Binaries, Velocity Jitter, and Rotation in Field Metal-poor Red Giant and Red Horizontal-Branch Stars
We summarize 2007 radial velocity measurements of 91 metal-poor fieldred giants. Excluding binary systems with orbital solutions, ourcoverage averages 13.7 yr per star, with a maximum of 18.0 yr. We reportfour significant findings. (1) Sixteen stars are found to bespectroscopic binaries, and we present orbital solutions for 14 of them.The spectroscopic binary frequency of the metal-poor red giants, with[Fe/H]<=-1.4, for periods less than 6000 days, is 16%+/-4%, which isnot significantly different from that of comparable-metallicity fielddwarfs, 17%+/-2%. The two CH stars in our program, BD -1°2582 and HD135148, are both spectroscopic binaries. (2) Velocity jitter is presentamong about 40% of the giants with MV<=-1.4. The twobest-observed cases, HD 3008 and BD +22°2411, showpseudoperiodicities of 172 and 186 days, longer than any knownlong-period variable in metal-poor globular clusters. Photometricvariability seen in HD 3008 and three other stars showing velocityjitter hints that starspots are the cause. However, the phasing of thevelocity data with the photometry data from Hipparcos is not consistentwith a simple starspot model for HD 3008. We argue against orbitalmotion effects and radial pulsation, so rotational modulation remainsthe best explanation. The implied rotational velocities for HD 3008 andBD +22°2411, both with MV<=-1.4 and R~50Rsolar, exceed 12 km s-1. (3) Including HD 3008and BD +22°2411, we have found signs of significant excess linebroadening in eight of the 17 red giants with MV<=-1.4,which we interpret as rotation. In three cases, BD +30°2034, CD-37°14010, and HD 218732, the rotation is probably induced by tidallocking between axial rotation and the observed orbital motion with astellar companion. But this cannot explain the other five stars in oursample that display signs of significant rotation. This high frequencyof elevated rotational velocities does not appear to be caused bystellar mass transfer or mergers: there are too few main-sequencebinaries with short enough periods. We also note that the lack of anynoticeable increase in mean rotation at the magnitude level of the redgiant branch luminosity function ``bump'' argues against the rapidrotation's being caused by the transport of internal angular momentum tothe surface. Capture of a planetary-mass companion as a red giantexpands in radius could explain the high rotational velocities. (4) Wealso find significant rotation in at least six of the roughly 15 (40%)red horizontal-branch stars in our survey. It is likely that theenhanced rotation seen among a significant fraction of both blue and redhorizontal-branch stars arose when these stars were luminous red giants.Rapid rotation alone therefore appears insufficient cause to populatethe blue side of the horizontal branch. While the largest projectedrotational velocities seen among field blue and red horizontal-branchstars are consistent with their different sizes, neither are consistentwith the large values we find for the largest red giants. This suggeststhat some form of angular momentum loss (and possibly mass loss) hasbeen at work. Also puzzling is the apparent absence of rotation seen infield RR Lyrae variables. Angular momentum transfer and conservation inevolved metal-poor field stars thus pose many interesting questions forthe evolution of low-mass stars.
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
| Detection of Metal-poor Stars in the Direction of the North Galactic Pole
A simple approach to detecting metal-poor stars is to measure amagnesium index, which depends on the Mg H band plus the three nearby Mgb lines and is derived through intermediate-band interference filters.An empirically established line of demarcation in the Mg index versusB-V diagram separates metal-poor stars from solar-abundance stars. Afurther separation between metal-poor dwarfs and giants depends on B-Vprimarily dwarfs for B-V<0.55, giants for B-V>0.7, with both dwarfsand giants falling in the transition region. For the metal-poor giantsthe distance from the demarcation line correlates well with [Fe/H],permitting estimates of stellar abundances. Stars in two regions on thesky in the vicinity of the north Galactic pole have been observed withsuch a set of filters. Eighteen stars (6% of the population of 299) inthe sample covering the V range 8.7 to 15.6 and 48 stars (31% of thepopulation of 163) in a deeper probe to V=19.9 found through thisprocess are suspected metal-poor stars according to their Mg indices.Twenty-three are specifically deemed giants, with<[Fe/H]><=-1.5.
| Neutron-Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal-poor Giants
New abundances for neutron-capture (n-capture) elements in a largesample of metal-poor giants from the Bond survey are presented. Thespectra were acquired with the KPNO 4 m echelle and coudé feedspectrographs, and have been analyzed using LTE fine-analysis techniqueswith both line analysis and spectral synthesis. Abundances of eightn-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, and Dy) in 43 stars havebeen derived from blue (λλ4070-4710, R~20,000, S/Nratio~100-200) echelle spectra and red (λλ6100-6180,R~22,000, S/N ratio~100-200) coudé spectra, and the abundance ofBa only has been derived from the red spectra for an additional 27stars. Overall, the abundances show clear evidence for a largestar-to-star dispersion in the heavy element-to-iron ratios. Thiscondition must have arisen from individual nucleosynthetic events inrapidly evolving halo progenitors that injected newly manufacturedn-capture elements into an inhomogeneous early Galactic halointerstellar medium. The new data also confirm that at metallicities[Fe/H]<~-2.4, the abundance pattern of the heavy (Z>=56) n-captureelements in most giants is well-matched to a scaled solar systemr-process nucleosynthesis pattern. The onset of the main r-process canbe seen at [Fe/H]~-2.9 this onset is consistent with the suggestion thatlow mass Type II supernovae are responsible for the r-process.Contributions from the s-process can first be seen in some stars withmetallicities as low as [Fe/H]~-2.75 and are present in most stars withmetallicities [Fe/H]>-2.3. The appearance of s-process contributionsas metallicity increases presumably reflects the longer stellarevolutionary timescale of the (low-mass) s-process nucleosynthesissites. The lighter n-capture elements (Sr-Y-Zr) are enhanced relative tothe heavier r-process element abundances. Their production cannot beattributed solely to any combination of the solar system r- and mains-processes, but requires a mixture of material from the r-process andfrom an additional n-capture process that can operate at early Galactictime. This additional process could be the weak s-process in massive(~25 Msolar) stars, or perhaps a second r-process site, i.e.,different from the site that produces the heavier (Z>=56) n-captureelements.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Abundances of metal-weak thick-disc candidates
High-resolution spectra of five candidate metal-weak thick-disc starssuggested by Beers & Sommer-Larsen are analysed to determine theirchemical abundances. The low abundance of all the objects has beenconfirmed, with metallicity reaching [Fe/H]=-2.9. However, for threeobjects the astrometric data from the Hipparcos catalogue suggest theyare true halo members. The remaining two, for which proper-motion dataare not available, may have disc-like kinematics. It is therefore clearthat it is useful to address properties of putative metal-weakthick-disc stars only if they possess full kinematic data. For CS22894-19 an abundance pattern similar to those of typical halo stars isfound, suggesting that chemical composition is not a useful discriminantbetween thick-disc and halo stars. CS 29529-12 is found to be C-enhancedwith [C/Fe]=+1.0 other chemical peculiarities involve the s-processelements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr],considerably larger than that found in more metal-rich carbon-richstars, but similar to those in LP 706-7 and LP 625-44, discussed byNorris et al. Hipparcos data have been used to calculate the spacevelocities of 25 candidate metal-weak thick-disc stars, thus allowing usto identify three bona fide members, which support the existence of ametal-poor tail of the thick disc, at variance with a claim to thecontrary by Ryan & Lambert.
| The effective temperature scale of giant stars (F0-K5). I. The effective temperature determination by means of the IRFM
We have applied the InfraRed Flux Method (IRFM) to a sample ofapproximately 500 giant stars in order to derive their effectivetemperatures with an internal mean accuracy of about 1.5% and a maximumuncertainty in the zero point of the order of 0.9%. For the applicationof the IRFM, we have used a homogeneous grid of theoretical modelatmosphere flux distributions developed by \cite[Kurucz (1993)]{K93}.The atmospheric parameters of the stars roughly cover the ranges: 3500 K<= T_eff <= 8000 K; -3.0 <= [Fe/H] <= +0.5; 0.5 <= log(g) <= 3.5. The monochromatic infrared fluxes at the continuum arebased on recent photometry with errors that satisfy the accuracyrequirements of the work. We have derived the bolometric correction ofgiant stars by using a new calibration which takes the effect ofmetallicity into account. Direct spectroscopic determinations ofmetallicity have been adopted where available, although estimates basedon photometric calibrations have been considered for some stars lackingspectroscopic ones. The adopted infrared absolute flux calibration,based on direct optical measurements of stellar angular diameters, putsthe effective temperatures determined in this work in the same scale asthose obtained by direct methods. We have derived up to fourtemperatures, TJ, TH, TK and T_{L'},for each star using the monochromatic fluxes at different infraredwavelengths in the photometric bands J, H, K and L'. They show goodconsistency over 4000 K, and there is no appreciable trend withwavelength, metallicity and/or temperature. We provide a detaileddescription of the steps followed for the application of the IRFM, aswell as the sources of error and their effect on final temperatures. Wealso provide a comparison of the results with previous work.
| Estimation of Stellar Metal Abundance. II. A Recalibration of the Ca II K Technique, and the Autocorrelation Function Method
We have recalibrated a method for the estimation of stellar metalabundance, parameterized as [Fe/H], based on medium-resolution (1-2Å) optical spectra (the majority of which cover the wavelengthrange 3700-4500 Å). The equivalent width of the Ca II K line (3933Å) as a function of [Fe/H] and broadband B-V color, as predictedfrom spectrum synthesis and model atmosphere calculations, is comparedwith observations of 551 stars with high-resolution abundances availablefrom the literature (a sevenfold increase in the number of calibrationstars that were previously available). A second method, based on theFourier autocorrelation function technique first described by Ratnatunga& Freeman, is used to provide an independent estimate of [Fe/H], ascalibrated by comparison with 405 standard-star abundances.Metallicities based on a combination of the two techniques for dwarfsand giants in the color range 0.30<=(B-V)_0<=1.2 exhibit anexternal 1 sigma scatter of approximately 0.10-0.20 dex over theabundance range -4.0<=[Fe/H]<=0.5. Particular attention has beengiven to the determination of abundance estimates at the metal-rich endof the calibration, where our previous attempt suffered from aconsiderable zero-point offset. Radial velocities, accurate toapproximately 10 km s^-1, are reported for all 551 calibration stars.
| Ca II H and K Photometry on the UVBY System. III. The Metallicity Calibration for the Red Giants
New photometry on the uvby Ca system is presented for over 300 stars.When combined with previous data, the sample is used to calibrate themetallicity dependence of the hk index for cooler, evolved stars. Themetallicity scale is based upon the standardized merger of spectroscopicabundances from 38 studies since 1983, providing an overlap of 122evolved stars with the photometric catalog. The hk index producesreliable abundances for stars in the [Fe/H] range from -0.8 to -3.4,losing sensitivity among cooler stars due to saturation effects athigher [Fe/H], as expected.
| On the Use of [Na/Fe] and [alpha/Fe] Ratios and Hipparcos-based (U, V, W) Velocities as Age Indicators among Low-Metallicity Halo Field Giants
We have examined the [Na/Fe] and [Mg/Fe] ratios in a sample of 68 fieldhalo giants with -3 <~ [Fe/H] <~ -1. We recalculated the Galactic(U, V, W) velocity components for these stars, using Hipparcos propermotions and a new Hipparcos-based distance scale. We used these data tosee how the abundance ratios may relate to kinematical substructure inthe Galactic halo. To isolate a set of true halo stars, we eliminatedmetal-weak thick-disk stars, about 10% of our sample. The field halogiants show the expected correlation of Na and Mg abundances, so we canuse Na as a surrogate for Mg and the alpha-elements. The most metal-poorstars show a wider dispersion of [Na/Fe] ratios than do the lessmetal-poor stars; the difference is most striking for stars onretrograde galactic orbits. Some 20% of our retrograde giants and 13% ofall our halo giants have [Na/Fe] <= -0.35 and may be significantlyyounger than the oldest halo objects. Halo giants considered ``young''by this Na abundance criterion show a preference for retrograde orbits.Giants in some globular clusters (e.g., M13) do not exhibit the Mgversus Na correlation found among halo field giants. Instead, they havevery large [Na/Fe] ratios and widely scattered [Mg/Fe] ratios, probablyinduced by deep mixing, which field halo giants apparently do notexperience.
| Broad-band JHK(L') photometry of a sample of giants with 0.5 > [Fe/H] > -3
We present the results of a three-year campaign of broad-band photometryin the near-infrared J, H, K and L' bands for a sample of approximately250 giant stars carried out at the Observatorio del Teide (Tenerife,Spain). Transformations of the Telescopio Carlos Sanchez systeminto/from several currently used infrared systems are extended to theredward part of the colour axis. The linearity of our photometric systemin the range -3 mag [Fe/H] >-3. Data of comparable quality previouslypublished have been added to the sample in order to increase thereliability of the relations to be obtained. We also provide mean IRcolours for giant stars according to spectral type.ables 1, 2 and 3 are only available in electronic form via the CDS(anonymous ftp 130.79.128.5 or http://cdsweb.u-strasbg.fr/Abstract.html
| Early evolution of the Galactic halo revealed from Hipparcos observations of metal-poor stars
The kinematics of 122 red giant and 124 RR Lyrae stars in the solarneighborhood are studied using accurate measurements of their propermotions obtained by the Hipparcos astrometry satellite, combined withtheir published photometric distances, metal abundances, and radialvelocities. A majority of these sample stars have metal abundances of(Fe/H) = -1 or less and thus represent the old stellar populations inthe Galaxy. The halo component, with (Fe/H) = -1.6 or less, ischaracterized by a lack of systemic rotation and a radially elongatedvelocity ellipsoid. About 16 percent of such metal-poor stars have loworbital eccentricities, and we see no evidence of a correlation between(Fe/H) and e. Based on the model for the e-distribution of orbits, weshow that this fraction of low-e stars for (Fe/H) = -1.6 or less isexplained by the halo component alone, without introducing the extradisk component claimed by recent workers. This is also supported by theabsence of a significant change in the e-distribution with height fromthe Galactic plane. In the intermediate-metallicity range, we find thatstars with disklike kinematics have only modest effects on thedistributions of rotational velocities and e for the sample at absolutevalue of z less than 1 kpc. This disk component appears to constituteonly 10 percent for (Fe/H) between -1.6 and -1 and 20 percent for (Fe/H)between -1.4 and -1.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Distribution and Studies of the Infrared Stellar Population in the Galaxy. VI. The Halo
We present infrared J, H and K observations of 69 local galactic halostars. We produce the two colour (JHK) and the colour magnitude (Kversus (J-K)) diagrams for this stellar sample and compare them with thesame diagrams for the stellar populations in the globular clusters M3,M13, M92 and 47 Tucanae and in the old open cluster M67; we also comparethese diagrams with those for the stellar population in the galacticbulge.
| The Infrared Ca II Triplet as a Metallicity Indicator of Stellar Populations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113.1066I&db_key=AST
| Sodium Abundances in Field Metal-Poor Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111.1689P&db_key=AST
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | בתולה |
| התרוממות ימנית: | 11h53m37.32s |
| סירוב: | -02°00'36.7" |
| גודל גלוי: | 9.611 |
| תנועה נכונה: | -22 |
| תנועה נכונה: | -116.5 |
| B-T magnitude: | 10.451 |
| V-T magnitude: | 9.681 |
קטלוגים וכינוים:
|
